The present invention relates to a process for the continuous production of polyurethane and polyurethane urea prepolymers by reaction of polyisocyanates with polyhydroxy compounds or polyamine compounds having molecular weights of 400 to 10,000. It is known that such prepolymers can be produced from polyisocyanates and polyhydroxyl and/or polyamino compounds. Various production processes for such prepolymers are known in the art.
Prepolymer production processes carried out in a reaction vessel of the type described in Ullmann's Encyklopadie der technischen Chemie, Vol. 19, Chap. 9.3, Verlag Chemie, Weinheim (1982) are attended by the disadvantage that the chemical and physical properties of the prepolymers (such as reactivity, degree of crosslinking and viscosity) change unfavorably during their reaction to polyurethane elastomers, so that uniform production is not possible.
Continuous production processes using mixing heads are also described in the same reference. However these processes have the disadvantage that mixing can only be carried out to a limited degree and only with starting materials of chemically the same type. Another problem is that it is difficult to seal the places where the shafts of the mixing elements pass through the housing of the mixing head against the internal pressure and the non-lubricating and sealing medium of the polyisocyanates. Accordingly, relatively high pressures cannot be used. A continuous process in which the prepolymers are produced in a twin-screw extrude with co-rotating screws and self-cleaning screw geometry at temperatures of 70 to 260.degree. C. is described in German Offenlegungsschrift 2,842,806. The disadvantages of this process are that the components are very thinly liquid at the temperatures mentioned, the mixing effect obtained by two co-rotating shafts is not great and the residence time is too short for a complete reaction.
German Offenlegungsschrift 2,302,564 mentions small-volume, stirred mixing chambers and mixing nozzles for mixing for the production of prepolymers. Mixing nozzles are known elements for mixing liquids. Detailed particulars of the geometry of the mixing nozzle and the arrangement of the volume flows or pressure conditions are not provided. In the Examples, a mix head rather than mixing nozzles is used for the production of prepolymers from polyisocyanates and polyol mixtures. A process using a mixing nozzle is not described.
German Offenlegungsschrift 2,823,762 describes a continuous process in which the starting components are passed through a static premixer, in which they are intermixed, the product temperature being so low that a reaction is largely avoided, thus preventing the products from caking. Although thorough mixing is achieved by this process, separation in the reaction zone is not prevented by suppression of the reaction at low temperatures. The prepolymer does not have sufficient reaction time in the following step to react out completely and uniformly.
European Patent 87,817 describes a process for the production of prepolymers in which the components are introduced into a static mixer comprising internally arranged heat exchangers. Optimal mixing is achieved by internally mounted mixing elements which also provide for a residue-free plug flow in the system. This process is only suitable for low-viscosity prepolymers and only for production runs with no change of raw materials. This is because residues (in some cases even over-crosslinked and brown in color) gradually accumulate in the mixing elements and, after the reaction to thermoplastic elastomers, give rise during their use to contamination of the end product in the event of product changes or to infusible particles in extrudates, such as films.
Accordingly, the problem addressed by the present invention was to provide a new process for the continuous production of polyurethane and polyurethane urea prepolymers by reaction of polyisocyanates with polyhydroxyl compounds and/or polyamine compounds having molecular weights of 400 to 10,000, in which
a) the components would be mixed and, at the same time, be very finely dispersed, PA1 b) rotating parts would not cause any sealing problems relative to the outside environment, PA1 c) sufficient reaction time would be available for a complete reaction, i.e. until the prepolymer has reacted with chain extenders, such as short-chain diols or diamines, to the thermoplastic polyurethane, PA1 d) the equipment used would be easy to clean and would not lead to any caking of product or blockages in the system.